High-frequency heating apparatus



1952 E. c WITSENBURG ETAL 2,623,176

HIGH-FREQUENCY HEATING APPARATUS Filed Aug. 31, 1949 INVENTORS HENDRIKADOLF TEUNISSEN EMlLlUS CAROLUS WITSEN URG Patented Dec. 23, 1952HIGHFREQUENCY HEATING APPARATUS Emilius Carolus Witsenburg and HendrikAdolf Teunissen, Eindhoven, Netherlands, assignors to Hartford NationalBank andTrust Company, Hartford, 001111., as trustee Application August31, 1949, Serial No. ll-3,312 .Inithe. Netherlands September 2, .1948

'8 Claims .Ffor the .inductive heating of metal articles orfor thecapacitative heating of articles consisting of non-conducting material,high-frequency heating apparatus is known, in which the high-frequencyenergy is taken from the output circuit of a regeneratively arrangedelectron discharge tube.

In order to obtain a satisfactory efiiciency it is known to use aclass-C electron-tube oscillator, the output circuit of which isincluded in the anode-circuit and from which a positive feedback voltageis taken. The positive feedback voltageis supplied, by way of a gridcondenser, to the control grid which is connected to the oathode by wayof a grid-leak resistance.

To obtain optimum efficiency it is necessary to match the loadresistance to the oscillator, in other words to ensure that the tube ismaximally controlled with respect to voltage and at the sametimemaximally controlled with respect to current. Both in the case ofinductive and capacitative coupling between oscillator and load, thecoupling. factor between oscillator and load may be variable formatching the load, limited matching being also possible by varying thepositive feedback of the tube oscillator.

If the high-frequency heating apparatus is used for heating articles ofdifferent kind and size, the power supplied to the load should beadjustable. To this .end it is usual tomake the supply voltageiof the.oscillator adjustable.

To indicate the oscillator load, use. may be made of a device indicatinganode current and grid current since, as is well known, the anodecurrentand grid current increase and decrease respectively with anincreasein load.

The present invention has for its object to provide an indication ofloadmatching in highfrequency heating apparatus of the said type, to permitadjustment to optimum load matching and/or .to avoid detrimentaloverloading of the oscillator tube.

According to the invention, this is achieved in a simple mannerinchigh-frequency heating apparatus ofthe aforesaid type. by using as anindicator of load-matching a difierential device which is controlled inopposite senses by the anode current and the grid current.

Thev 'difierential device preferably consists of apointer device with acentral zero position.

lfa differential pointer device is chosen such that in a normal, butotherwise arbitrary Working condition and optimum load matching, theinfluences exerted by the anode current and the grid current compensateoneanother, it is surprisingly found that the indication obtainedconsistently indicates practically optimum load matching in Workingconditions varying Within wide limits, such as occur upon variation of,say, the oscillator supply voltage, the coupling between oscillator andload, the oscillator backcoupling or a combined variation of'thesefactors. Upon wrong matching the direction of deviation of the pointerdevice indicates undermatching or overmatching.

The pointer device may-comprise scales which are calibrated in terms ofthe power supplied to the load for different oscillator supply voltages.

According to a further feature of the invention, a differential devicemay also be used, whilst retaining the aforesaid advantages, to indicateor prevent detrimental overloading of the oscil-. lator tube. For thispurpose the differential device preferably consists of a differentialrelay functioning as a maximum relay having a normally closedcontactplaced in the anode supply circuit. 7

In order that the invention may be more clearly understood and readilycarried into effect, one example will now be described with reference tothe accompanying drawing.

Fig. l is a schematic diagram of one preferred embodiment in accordancewith they invention; and

Fig. 2 11s a schematic diagram of a modification of the circuit showninFig. 1.

In the circuit arrangement. shown in Fig. 1, the high-frequency energyis taken from a class- C connected electron tube oscillator comprising adirectly heated triode I.

In the anode circuit of the electron tube oscillator, consisting of aColpitts arrangement, the frequency determining oscillatory circuitcomprising a coil 2 and series-connected .condensers 3, 4 is connectedto the anode 5 of the triode by way of a blocking condenser 5. Thepositive feedback voltage is taken from the capacitative voltage dividerformed by the .condensers 3, 4 and is supplied, by way of agridcondenser I, to the control grid l0 which is connected to the cathode 9by way of a grid resistance 8.

The circuit current appearing in the oscillatory circuit uponoscillation of the circuit arrangement is used for heating a workpiece lI. For this purpose a part I? of coil 2 is inductively coupled with acoupling coil l3 which usually consists of a single turn and isconnected to a work coil I 4.

For optimum load matching, the coil part I! is furnished with tappingpoints 12f. so that the turns ratio of the transformer constituted bycoil part l2 and coupling coil I3 is adjustable in steps. Furthermore,load matching is obtainable by placing an auxiliary coil IS in the anodecircuit of the oscillator tube, which coil, together with the parts ofcoil 2 not forming the coupling coil I2, constitute an autotransiormerhaving a variable coupling factor. This may be effected by arranging anaxially movable coupling coil l5 concentrically in coil 2.

The supply voltage of the oscillator circuit arrangement is obtained byfull wave rectification, by means of rectifiers l6, ll, of analternating voltage of mains frequency which is taken from the secondaryof a transformer 18. This direct voltage is supplied, by way of chokesI9, l9, to the anode 6 of triode I. The primary of the transformer i8 isprovided with tapping points 20, so that the supply voltage is variable,in order that the power to be supplied to the load may be variable.

In order that, with greatly varying working conditions, optimum loadmatching is consistently possible and/or detrimental overloading of theoscillator tube may be avoided, a differential pointer device is usedand is controlled in opposite senses by the anode-current and thegridcurrent.

This may consist of a differential device comprising two coils connectedin the anode-circuit and grid-circuit respectively, but it is preferredto use, as in the present example, a combination of resistances 2| and22, shunted by a pointer device 23 having a central zero position, andconnected in the grid-circuit and anode-circuit respectively. Theresistances 2| and 22 are chosen such that in the case of optimum loadmatching, the pointer of the device 23 is in the central zero position.

In the case of optimum load matching, the indication is consistentlypractically zero and independent of the working conditions. In the caseof wrong matching a deviation to the one side indicates undermatching,whereas a deviation to the other side indicates overmatching also withrespect to the value. The indication with respect to load matching ismore accurate as the supply voltage increases.

In the circuit arrangement shown, the feedback factor of the oscillator,upon variation of the transformation ratio, varies in such manner as tocounteract optimum load matching. This may be prevented by using acurrent dependent resistance acting as a grid-resistance 8.

Some data for circuit components which have been found satisfactory inpractice, for use in the circuit shown in the drawing, are given below:

The electron tube oscillator comprises two parallel-connected Philipstransmission triodes type TA 12/20 Maximum power supplied, 20 kilowattsMaximum anode voltage, 10 kilowatts Maximum anode current per tube, 1.9amp.

Maximum grid current per tube, 0.5 amp.

Pointer device: voltmeter with central zero position and a maximumdeviation at approximately 5 volts.

Value of the resistance 22 placed in the anodecircuit 1 ohm.

Value of the resistance 2| placed in the gridcircuit: 7 ohms.

The high-frequency furnace used in practice should have a great workingsecurity. For this purpose it is known to connect in the cathodecircuita resistance 24 providing a negative gridbias by which overloading iscounteracted. In order to reduce the possibility of detrimentaloverloading, however, in the case of abnormal handling of the heatinginstallation, as shown in Fig. 2, a diiferential relay 25 functioning asa maximum relay may be used as a differential device, which relaycomprises a normally closed contact 26 placed in the anode supplycircuit.

A device of this type used for preventing overloading permits the use ofa smaller cathode resistance or even the omission of this resistance,which results in a higher efficiency of the heating installation.

The invention may also be used in oscillator arrangements other thanthat shown in the drawing, for example in tube oscillators, in which thefeedback voltage is taken from a feedback coil which is placed in thegrid-circuit and is inductively coupled with the frequency determiningoscillatory circuit included in the anode-circuit, and may also be usedin oscillation arrangements with capacitative coupling of the load.

What we claim is:

1. A high-frequency heating apparatus comprising a class C electron tubeoscillator, having a variable supply voltage and an output circuitincluded in the anode circuit, from which output circuit a positivefeedback voltage is taken and supplied, by way of a grid condenser, to acontrol grid connected to the cathode by way of a grid leakageresistance, and furthermore comprising means for load matching,characterised in that a differential device controlled in oppositesenses by anode current and grid current is provided for indicatingload-matching.

2. A high-frequency heating appa-artus as claimed in claim 1,characterised in that the differential device comprises two coils whichare connected in the anode-circuit and grid-circuit respectively.

3. A high-frequency heating apparatus as claimed in claim 1characterised in that both the grid circuit and the anode circuit of theoscillator comprise a resistance, which resistances are shunted by thesaid differential device.

4. A high-frequency induction heating system comprising a class Coscillator including an electron discharge tube having a cathode, a gridand an anode, a resonant output circuit coupled to said anode, acondenser regeneratively coupling said output circuit to said grid,means to couple said output circuit to a load, and means to supplyoperatin potentials to the electrodes of said tube to sustainoscillations therein whereby an increase in load results in an increasein anode current and a decrease in grid current; and apparatus forindicating optimum matching of said oscillator to said load includingmeans coupled to the grid of said tube to develop a first voltageproportional to said grid current, means coupled to the anode of saidtube to develop a second voltage proportional to said anode current, adifferential indicator, and means to apply said first and secondvoltages in opposing senses to said indicator.

5. An arrangement, as set forth in claim 4, wherein said differentialindicator comprises first and second coils to which are applied saidfirst and second voltages respectively, and a zero center pointeractuated by said coils and occupying a zero position when the voltagesapplied thereto are equal.

6. An arrangement. as set forth in claimli,

wherein said first and second Voltages are of equal amplitude in theoptimum load matching condition of said oscillator.

7. An arrangement, as set forth in claim 6, wherein said indicatorfurther includes a plurality of calibrated scales corresponding topredetermined supply potentials.

8. A high-frequency induction heating system comprising a class Coscillator provided With an electron discharge tube having a cathode, agrid and an anode, a resonant output circuit coupled to said anode, acondenser regeneratively coupling said output circuit to said grid,means to couple said output circuit to a load, and means to supplyoperating potentials to the electrodes of said tube to sustainoscillations herein whereby an increase in load results in an increasein anode current and decrease in grid current; and apparatus forindicating matching of said oscillator to said load including meanscoupled. to the grid of said tube to develop a first voltageproportional to said grid current, means coupled to the anode of saidtube to develop a second voltage proportional to said anode current andequal to I said first voltage in the optimum matching condition of saidoscillator, a differential relay, means to apply said first and secondvoltages in opposing directions to said relay whereby said relay isactuated When said second voltage exceeds said first voltage, and meansresponsive to the actuation of said relay to interrupt said supplypotentials.

EMILIUS CAROLUS WITSENBURG. HENDRIK ADOLF TEUNISSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

